As a digital-signal transmission system, OFDM (Orthogonal Frequency Division Multiplexing) has been proposed in recent years. In the OFDM system, data is transmitted employing a plurality of carriers that are orthogonal to each other in the frequency domain. For this reason, an OFDM transmitter modulates a transmission signal using IFFT (Inverse Fast Fourier Transform), and an OFDM receiver demodulates the transmission signal using FFT (Fast Fourier Transform). Since the OFDM system has high frequency efficiency, its application to digital terrestrial broadcasts has been widely explored. In Japan, the digital terrestrial broadcasting system called ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) has adopted the OFDM.
FIG. 1 is a diagram illustrating the basic configuration of a general OFDM receiver. In FIG. 1, an OFDM signal received via an antenna is provided to a tuner 1. The tuner 1 selects a signal in a desired channel from the received signal. An A/D conversion unit 2 converts the signal selected by the tuner 1 into a digital signal. The digital signal is converted into a complex baseband signal by an orthogonal demodulation unit 3. The complex baseband signal is converted into a frequency-domain signal by an FFT unit 4. As a result, a plurality of signals transmitted using a plurality of carriers having different frequencies are obtained. An OFDM signal for digital broadcasting contains, for example, a data signal, a scattered pilot (SP) signal, an auxiliary channel (AC) signal, and a transmission and multiplexing configuration control (TMCC) signal.
The data signal and SP signal are provided to the transmission path equalization unit 5. The SP signal is a known signal for which the transmission phase and transmission power have been determined in advance. The transmission path equalization unit 5 equalizes the data signal using the SP signal. A deinterleave unit 6 performs a deinterleave process for the output data from the transmission path equalization unit 5. The recovered data are output in the transform stream (TS) format after a correction process is performed by an error correction unit 7.
In Japan, digital TV broadcast (13ch-62ch) using the UHF band and digital radio broadcast (7ch, 8ch) are specified as the digital terrestrial broadcast (ISDB-T). For the digital TV broadcast, a 6 MHz band is assigned to each channel, and each of the bands is further divided into 13 segments. Broadcasting for a general TV set (fixed terminal) is performed using 12 segments of the 13 segments (the broadcasting is sometimes called “full-segment broadcasting”), and broadcasting using the remaining one segment (the broadcasting is generally called “one-segment broadcasting”) is performed for a mobile terminal.
A transmitting station multiplexes and transmits an A-layer TS for the one-segment broadcasting and a B-layer TS for the full-segment broadcasting simultaneously. At this time, the same contents are distributed with the one-segment broadcasting and the full-segment broadcasting (although the amount of information is different between the two types of broadcasting). In other words, simultaneous broadcasting is carried out. A digital broadcast receiver usually receives either one of the one-segment broadcasting and the full-segment broadcasting.
However, a receiver that can receive both the one-segment broadcasting and the full-segment broadcasting has been implemented. Such a receiver is equipped with, as illustrated in FIG. 2, an output layer selecting unit 8 for selecting one of A-layer TS and B-layer TS in accordance with the bit error rate (BER) of received data.
A system has been known as a related art, in which a signal in a partial layer is multiplexed into a plurality of segments and transmitted. According to this system, the signal is received by selection diversity with which the segment having the best reception state is selected from the plurality of segments in which the signal of the partial layer is multiplexed, or by combining diversity with which the signals in the multiplexed segments of the signal in the partial layer are combined.
As another related art, a digital broadcast receiving apparatus that can receive both the 12-segment broadcasting and one-segment broadcasting simultaneously and selectively has been known. The digital broadcast receiving apparatus includes a display switching unit that selectively switches and outputs, a first video image obtained by a 12-segment video image decoding unit and a second video image obtained by a one-segment video image decoding unit to a first display unit and a second display unit, respectively.
Further, a communication system has been known as another related art, in which required data can be selected from hierarchized transmitted data in accordance with the reception state. A receiving apparatus in the system include an information layer decision unit that decides the layer in the hierarchy to which the data transmitted from a transmission apparatus belongs to. A hierarchized data receiving unit receives the data in the layer decided by the information layer decision unit while limiting or selecting the data in accordance with the reception capacity or the propagation environment.
These arts are disclosed in, for example, Japanese Patent Application Publications No. 2006-20128, No. 2007-74092, and No. 2004-128988.
In the conventional arts, the video image is interrupted when the receiver switches between the one-segment broadcasting and the full-segment broadcasting. In addition, in a receiver that switches between one-segment/full-segment in accordance with the error rate of received data, a temporary deterioration in the error rate can trigger the switching process under fading or multipath environment, causing frequent occurrence of unnecessary switching processes.
Therefore, there has been a need for developing a receiver and receiving method with which switching between a plurality of hierarchical layers in digital broadcast can be performed seamlessly.